In a conventional mechanical watch, air friction on the balance wheel, the friction of the pivots in their bearings and the reaction forces of the balance spring stud limit the quality factor of the resonator. It is sought to eliminate pivot friction and the forces of reaction at the point of attachment.
In a watch, the watch movement must have optimal isochronism in all positions in space, which involves designing movements capable of compensating for the effects of gravity on their components.
Prior art documents describe oscillators comprising several primary resonators having flexible branches, arranged with respect to each other such that their errors are averaged out.
A first type of oscillator with coupled primary resonators is known in the form of a U-shaped tuning fork wherein each branch is formed by a primary resonator; however, this system is very sensitive to changes of position in space.
CH Patent 451021 in the name of Ebauches SA thus describes a symmetrical U-shaped oscillator with two flexible branches that vibrate in tuning fork mode, each being connected to a stiff arm forming a counterweight, and each primary resonator thereby formed is arranged such that the instantaneous centre of rotation coincides with the centre of gravity, such that the oscillator frequency hardly varies when the position of the centre of gravity changes. Changing to a U-shaped design with extended branches proves better than the U-shape of the prior art. However, the instantaneous centre of rotation moves continuously during the oscillation of each primary resonator.
CH Patent 46203, also in the name of Ebauches SA is a variant of the preceding Patent, comprising a counting device that transforms the oscillating motions of one of the two resonators into rotating motions of a counting wheel; the counting device is attached to one of the stiff arms, such that the counting device is not affected by accelerations and particularly by shocks.
GB Patent 1293159 in the name of SEIKO develops a theory based on the effect on rate regularity of the displacement derivative of the centre of mass with respect to the angle of rotation, and attempts to obtain a straight line displacement of the centre of mass of each primary resonator, to optimise the effect on rate. To this end, the centre of mass is positioned two thirds of the way along the flexure strip used in this system, in order theoretically to cancel out the effect on rate in vertical positions. However, the centre of mass moves a great deal, and this system is still sensitive to shocks. Further, this theory is based on a geometric approximation, since in reality the deflected shape of the flexible strip is no longer really an arc of a circle, and the assumed rectilinear displacement of the centre of mass is not confirmed.
U.S. Pat. No. 3,192,702A in the name of Yoshiaki Kato discloses a mechanical oscillator for a time base, with two symmetrical resonators coupled on either side of a core from which each is suspended by a flexible strip.
FR Patent Application 1605076A in the name of the Straumann Institute discloses a mechanical oscillator with a torsion bar forming the elastic element. The ends of this bar bear weights having the same moment of inertia with respect to the axis of the bar which has median symmetry, such that the oscillations of one half of the bar are transmitted to the other half, with the ends oscillating in phase opposition.
U.S. Pat. No. 3,277,394A in the name of William Holt discloses a temperature compensated electromechanical resonator, with two rings oscillating in phase opposition.
U.S. Pat. No. 3,318,087A in the name of Robert Favre, Movado, discloses a torsional oscillator, with a spring delivering a torque, integral with a frame and an oscillating weight, and cut to release elastic elements perpendicular to its torsional axis, arranged and dimensioned to work substantially by bending.